Molecular sinkers: X-ray photoemission and atomistic simulations of benzoic acid and benzoate at the aqueous solution/vapor interface.

In this work, we provide a detailed microscopic picture of the behavior of benzoic acid at the aqueous solution/vapor interface in its neutral as well as in its dissociated form (benzoate). This is achieved through a combination of highly surface-sensitive X-ray photoelectron spectroscopy experiments and fully atomistic molecular simulations. We show that significant changes occur in the interface behavior of the neutral acid upon release of the proton. The benzoic acid molecules are found to be strongly adsorbed at the interface layer with the planes of the aromatic rings oriented almost parallel to the water surface. In contrast, in the benzoate form, the carboxylate group shows a sinker-like behavior while the aromatic ring acts as a buoy, oriented nearly perpendicular to the surface. Furthermore, a significant fraction of the molecular ions move from the interface layer into the bulk of the solution. We rationalize these findings in terms of the very different hydration properties of the carboxylic group in the two charge states. The molecule has an amphiphilic nature, and the deprotonation thus changes the hydrophobic/hydrophilic balance between the nonpolar aromatic and the polar carboxylic parts of the molecule. That, consequently, leads to a pronounced reorientation and depletion of the molecules at the interface.

Salting out in organic solvents: a new route to carbon nanotube bundle engineering.

In this study we investigate salt effects on bundle formation of carbon nanotubes (CNTs) dispersed in an organic solvent, N-methyl-2-pyrrolidone (NMP). Addition of NaI salt leads to self-assembly of CNTs into well-recognizable bundles. It is possible to control the size of the CNT bundles by varying the salt concentration.

Selective Na(+)/K(+) effects on the formation of α-cyclodextrin complexes with aromatic carboxylic acids: competition for the guest.

We investigated the effects of K(+) and Na(+) ions on the formation of α-cyclodextrin complexes with ionized aromatic carboxylic acids. Using solution calorimetry and (1)H NMR, we performed the thermodynamic and structural investigation of α-cyclodextrin complex formation with benzoic and nicotinic acids in different aqueous solutions containing K(+) and Na(+) ions as well as in pure water. The experiments show that the addition of sodium ions to solution leads to a decrease in the binding constants of the carboxylic acids with α-cyclodextrin as compared to pure water and solutions containing potassium ions. From another side, the effect of potassium ions on the binding constants is insignificant as compared to pure water solution. We suggest that the selectivity of cation pairing with carboxylates is the origin of the difference between the effects of sodium and potassium ions on complex formation. The strong counterion pairing between the sodium cation and the carboxylate group shifts the equilibrium toward dissociation of the binding complexes. In turn, the weak counterion pairing between the potassium cation and the carboxylate group has no effect on the complex formation. We complemented the experiments with molecular modeling, which shows the molecular scale details of the formation of cation pairs with the carboxylate groups of the carboxylic acids. The fully atomistic molecular simulations show that sodium ions mainly form direct contact pairs with the carboxylate group. At the same time, potassium ions practically do not form direct contact pairs with the carboxylate groups and usually stay in the second solvation shell of carboxylate groups. That confirms our hypotheses that the selective formation of ion pairs is the main cause of the difference in the observed effects of sodium and potassium salts on the guest-host complex formation of α-cyclodextrin with aromatic carboxylic acids. We propose a molecular mechanism explaining the effects of salts, based on competition between the cations and α-cyclodextrin for binding with the ionized carboxylic acids.